1. Field of the Invention
The present invention relates to an air conditioning system suitable for use in vehicles, and more particularly to an air conditioning system for a cooling apparatus having a photoelectric detecting device for detecting a state of refrigerant circulating in a refrigerant circuit.
2. Description of the Prior Art
A typical conventional air conditioning system for vehicles is shown in FIGS. 8 to 10. In FIG. 8, a refrigerant such as freon gas is circulated in refrigerant circuit 1 formed from pipe 2. Compressor 3, condenser 4, expansion valve 8 and evaporator 5 are provided in refrigerant circuit 1 sequentially in the circulation direction of the refrigerant which is shown by arrows "A". The endothermic surface of evaporator 5 is exposed to the interior of the vehicle (not shown). After the refrigerant is compressed by compressor 3, the refrigerant is transformed in phase from a high-pressure gas to a high-pressure liquid in condenser 4 and further to a low-pressure gas as it passes through expansion valve 8 and evaporator 5. When the refrigerant is transformed from a liquid phase to a gaseous phase (vapor phase) by evaporator 5, the refrigerant absorbs heat from the interior of the vehicle and the vehicle interior is cooled. Expansion valve 8 is provided between condenser 4 and evaporator 5. Expansion valve 8 reduces the pressure of the refrigerant to a relatively low pressure so that the liquefied high-pressure refrigerant can be easily vaporized when it passes through evaporator 5.
A receiver tank 6 is provided in refrigerant circuit 1 at a position between condenser 4 and expansion valve 8. Receiver tank 6 temporarily stores refrigerant "F" which has been transformed to a liquid phase. On top of receiver tank 6, an inspection hole 6A is provided for observing the liquefaction of the refrigerant. Receiver tank 6 is connected to introduction pipe 2A and extraction pipe 2B constituting parts of pipe 2, as shown in FIG. 9. A desiccator 7 is provided in receiver tank 6. After the water component of the refrigerant introduced into receiver tank 6 through introduction pipe 2A is removed by desiccator 7, the refrigerant is stored in the receiver tank as a liquefied refrigerant. The liquefied refrigerant is sent to expansion valve 8 through extraction pipe 2B as shown by arrow "A".
An optical type (photoelectric) flow sensor 9 is provided in refrigerant circuit 1 at a position between receiver tank 6 and expansion valve 8 as a refrigerant state detecting device. Flow sensor 9 comprises an emitter 9A for emitting a light into pipe 2B and a receiver 9B for receiving the light transmitted through the pipe. Emitter 9A and receiver 9B are provided on the pipe aligned with each other. Flow sensor 9 detects a phase of the refrigerant passing through pipe 2B, i.e., whether the refrigerant is in a liquid phase.
A determination circuit 10 is coupled to flow sensor 9. Determination circuit 10 is provided for determining a phase of the refrigerant passing through pipe 2B according to a signal sent from flow sensor 9. Determination circuit 10 is shown in FIG. 10. Determination circuit 10 comprises a constant voltage generation circuit 11, a detection circuit 16 coupled to flow sensor 9, an amplification circuit 20, a comparison circuit 24 and an information output circuit 30.
Constant voltage generation circuit 11 is coupled to the plus side "B" of a battery (not shown). Constant voltage generation circuit 11 comprises input compensation capacitor 13 and three terminal regulator 14 which are coupled between plus terminal "B" and ground 12. Output compensation capacitor 15 is coupled between the output of three terminal regulator 14 and ground. Constant voltage generation circuit 11 outputs a constant voltage VCC (for example 5 V).
Detection circuit 16 comprises resistor 17 for controlling a current of emitter 9A, coupled in series to the emitter 9A between the circuit at a voltage of VCC and ground 12, and resistor 18 for controlling a detecting signal of receiver 9B, coupled in series to the receiver 9B between the circuit at the voltage of VCC and ground 12. Junction 19 between receiver 9B and resistor 18 is coupled to amplification circuit 20. Receiver 9B controls a current flowing in accordance with a transmittance of the light transmitted through pipe 2B. A detection voltage "V" controlled in accordance with the above current and the resistance of resistor 18 is output from junction 19 as a detection signal.
Amplification circuit 20 comprises operational amplifier 21, resistor 22 having a resistance of R1 coupled between the inverting terminal of the operational amplifier and ground 12, and feedback resistor 23 having a resistance of R2 coupled between the inverting terminal and the output terminal of the operational amplifier. The non-inverting terminal of operational amplifier 21 is coupled to junction 19 of detection circuit 16. Amplification circuit 20 amplifies the voltage "V" output from detection circuit 16 in accordance with the resistances of R1 and R2 of resistors 22 and 23 at amplification factor "A" calculated by the following equation. EQU A=(R1+R2)/R1
Output voltage "E" of amplification circuit 20 is calculated by the following equation. EQU E=A.times.V
Comparison circuit 24 comprises operational amplifier 28, and potential dividing resistors 25 and 26 having resistances of R3 and R4, respectively, coupled in series between the circuit at the voltage of VCC and ground 12. Junction 27 between resistors 25 and 26 is coupled to the inverting terminal of operational amplifier 28. Output voltage "E" of amplification circuit 20 is input into the non-inverting terminal of operational amplifier 28. Reference voltage Vi is set by the respective resistances of resistors 25 and 26 as a reference value calculated by the following equation. EQU Vi=(R3/(R3+R4)).times.VCC
Output voltage "E" which is input into the non-inverting terminal of operational amplifier 28 and reference voltage Vi set for the inverting terminal thereof are compared in the operational amplifier. If Vi&lt;E, a determination voltage V0 is output as a high-level signal, and if Vi&gt;E, then the determination voltage V0 is output as a low-level signal.
Information output circuit 30 is provided for indicating or warning a state of refrigerant charged in refrigerant circuit 1. Information output circuit 30 comprises input protective resistor 31, protective resistor 32 coupled between the input protective resistor 31 and ground 12, NPN-type transistor 33 having a switching function whose base is coupled to the input protective resistor 31, and light emitting diode 35 coupled to VCC terminal via control resistor 34 and coupled to the collector of the transistor. The emitter of transistor 33 is coupled to ground 12. Light emitting diode 35 is placed in, for example, an engine compartment or the interior of a vehicle, and functions as an information lamp. If a high-level determination voltage V0 is output from comparison circuit 24, a current flows between the base and emitter of transistor 33 via input protective resistor 31, and current flows between the collector and emitter of transistor 33. Diode 35 emits a light by the turning "on" of transistor 33.
In such a conventional air conditioning system, determination circuit 10 operates, for example, as shown in FIG. 11, when refrigerant is charged into refrigerant circuit 1.
When the amount of charged refrigerant has not yet reached a proper amount, the refrigerant contains a refrigerant of a vapor phase (bubbles) at a position of pipe 2B where flow sensor 9 is provided and the refrigerant should be in a liquid phase if the amount of charged refrigerant reaches a proper amount. Since the transmissibility of a light through the refrigerant of a mixing phase is low, the amount of a light detected by receiver 9B is small. Therefore, output voltage "V" output from detection circuit 16 is low. The output voltage "V" is amplified to a voltage "E" at an amplification factor "A" by amplification circuit 20. The output voltage "E" from amplification circuit 20 is compared with reference voltage Vi in comparison circuit 24. Until output voltage "E" becomes higher than reference voltage Vi, determination voltage V0 output is a low-level signal. As a result, transistor 33 does not turn "on", and light emitting diode 35 does not operate, thus, indicating that the amount of charged refrigerant has not yet reached a proper amount.
When the amount of charged refrigerant has reached a proper amount, the refrigerant present at flow sensor 9 in pipe 2B indicates a complete liquid phase. Namely, refrigerant of a vapor phase does not exist. Therefore, the transmission of a light through the refrigerant increases and is detected by receiver 9B. Output voltage "V" output from detection circuit 16 becomes high, and a high output voltage "E" amplifying the output voltage "V" at an amplification factor "A" is output from amplification circuit 20. The output voltage "E" from amplification circuit 20 is compared with reference voltage Vi in comparison circuit 24. When output voltage "E" exceeds reference voltage Vi, a high-level determination voltage V0 is output from comparison circuit 24. Transistor 33 of information circuit 30 turns "on", and light emitting diode 35 emits a light, indicating that the amount of charged refrigerant has reached a proper amount.
Flow sensor 9 continues to detect a phase of refrigerant present in refrigerant circuit 1 at a position of the exit side of receiver tank 6, after the system is charged with refrigerant. If the amount of refrigerant in refrigerant circuit 1 decreases, for example, from leakage of the refrigerant, the amount of the refrigerant present in the refrigerant circuit gradually decreases, and bubbles of refrigerant are generated. The mixing state of liquid and vapor phases of the refrigerant is detected by flow sensor 9, and output voltage "E" of detection circuit 16 decreases as shown by the characteristic curve "S1" in FIG. 11 and the output of a high-level determination voltage V0 is stopped. The operation of light emitting diode 35 is stopped indicating insufficient amount of refrigerant in the system.
In the conventional air conditioning system functioning in such a manner, however, there are the following problems.
When a refrigerant containing a lubricating oil is charged into refrigerant circuit 1 while compressor 3 is driven, the lubricating oil separates from the refrigerant depending upon temperature, and a part of the lubricating oil insoluble to the refrigerant sometimes turns translucent, or muddy white, in the refrigerant circulating in pipe 2. Since the lubricating oil circulates in refrigerant circuit together with the refrigerant, when the translucent lubricating oil passes through the position provided with flow sensor 9, the light emitted from emitter 9A is partially interrupted by the lubricating oil and the transmitted light received by receiver 9B decreases. Therefore, the characteristic curve of output voltage "E" greatly varies as shown by areas "S2" and "S3" in FIG. 11 when the translucent lubricating oil passes through flow sensor 9, even after the amount of the charged refrigerant has reached a proper amount. As a result, output voltage "E" does not precisely correspond to the state of the refrigerant circulating in refrigerant circuit 1. This output voltage "E" is compared with reference voltage Vi. When output voltage "E" becomes lower than reference voltage Vi, the output of a high-level determination voltage V0 is stopped, and when output voltage "E" becomes higher than reference voltage Vi again, a high-level determination voltage V0 is output again, as shown in FIG. 11. Thus, the output of a high-level determination voltage V0 is repeated. Since the flashing of light emitting diode 35 is repeated in accordance with the variation of determination voltage V0, whether or not the amount of refrigerant is proper be cannot accurately determined.